1. Field of the Invention
This invention broadly relates to a process for the catalytic conversion of methanol to formaldehyde. In particular, the present invention relates to a process for preparing formaldehyde from methanol by contacting methanol and oxygen in contact with a first catalyst bed containing a silver catalyst, and passing the effluent from the first conversion into contact with a second catalyst bed containing an elemental copper catalyst.
2. Description of Related Art
There are primarily two commercially accepted processes for converting methanol to formaldehyde. The first utilizes a silver catalyst and operates in an oxygen lean atmosphere. The second utilizes a metal oxide catalyst and operates in a methanol lean atmosphere. The first process involves passing a mixture of methanol vapor and air over a fixed bed silver catalyst at approximately atmospheric pressure and absorbing the product gases in water. The mechanism by which methanol is converted to formaldehyde is believed to be a combination of two reactions separately involving the dehydrogenation and partial oxidation of methanol: EQU CH.sub.3 OH.fwdarw.HCHO+H.sub.2 (dehydrogenation) EQU CH.sub.3 OH+1/2O.sub.2 .fwdarw.HCHO+H.sub.2 O (partial oxidation)
Both single stage and multiple stage silver-catalyzed processes are known. Single stage operation is widely used but generally suffers from the disadvantage that rather high amounts of unconverted methanol are contained in the desired formaldehyde product emerging from the catalyst bed. For many applications, methanol is an undesirable contaminant in the formaldehyde product and must be separated from the recovered formaldehyde solution. Such recovery often entails a substantial investment in distillation facilities and energy to carry out such separations. Formaldehyde product specifications requiring a methanol content of no greater than 2% by weight are not uncommon.
One technique that has been suggested for eliminating the need for facilities to distill off methanol is sequential silver catalyst stages with interstage cooling. A basic two-stage catalytic process is disclosed in Meath U.S. Pat. No. 2,462,413.
Northeimer U.S. Pat. No. 3,959,383, purports to be an improvement of the Meath process involving the use of higher space velocities in the second stage converter and the employment of silver crystals of a specific particle size as the catalyst. These changes purportedly yield even lower amounts of methanol in the formaldehyde product.
Other two-stage processes have been described in the art. Payne, U.S. Pat. No. 2,519,788, describes a process for converting methanol to formaldehyde using an adiabatic first stage reactor for conducting methanol partial oxidation and dehydrogenation and containing a silver catalyst and an isothermal second stage reactor for conducting methanol oxidation and containing a metal oxide catalyst.
McClellan et al, in U.S. Pat. No. 3,987,107, disclose a two step, vapor phase process for converting methanol to formaldehyde. In the first step, methanol is contacted with a silver catalyst, while in the second step the remaining methanol, after inter-stage cooling and the introduction of auxiliary air, is contacted with a bismuth molybdate or bismuth phosphomolybdate-on-titania catalyst.
Efforts continue to be made to find new ways for further improving the conversion of methanol to formaldehyde without compromising selectivity. The present invention provides a two step catalytic process using known catalysts that allows such improvements to be obtained.